The present invention relates to a monitor mechanism for use with a scanning optical system such as a laser printer which draws patterns by scanning with laser light. This mechanism is used to detect the position of scanning by a light beam.
In conventional scanning pattern drawing apparatuses such as laser printers, a photodetector is provided in the scanning optical system at a position that is optically conjugate with the drawing surface and which is ahead of the letter in the direction of beam tracing, and the output produced by the photodetector when it is traversed by light beam is used as a vertical sync signal for the scanning optical system in order to control operations such as READ timing.
However, this arrangement is not capable of attaining a high precision of pattern drawing due to the lack of any way of correcting the timing of pattern drawing operations within a scanning field or sub-field so as to correct, for example, for fluctuations in the speeds of system motors.
With a view to solving this problem, it has been proposed that a monitor beam be employed in addition to the pattern drawing beam to perform simultaneous scanning with the pattern drawing beam. The position of scanning with the monitor beam is detected at a position optically equivalent (conjugate) to the drawing surface to insure constant detection of the position where the drawing beam is describing a pattern on said surface.
In the conventional method, however, the pattern drawing and monitor beams are projected in different directions into a deflector and a scanning lens so that the two beams are spatially isolated, with the monitor beam being reflected by a mirror to be guided into a detection system. Because of this arrangement, if the difference in the position of incidence into the scanning lens causes such an adverse effect as operational errors, the position of scanning by the pattern drawing beam cannot be correctly detected from the position of the monitor beam, which presents a substantial problem if patterns must be described with high precision.